1. Field of the Invention
This invention relates to technology for detecting a leading edge and/or a trailing edge of a piece of a recording medium, suitable for use in a recording apparatus such as either a serial recording apparatus wherein a recording head is moved in the direction across the width of a piece of a recording medium and printing is carried out or a page recording apparatus which transfers a toner image formed on a photoconductive drum to a piece of a recording medium.
2. Description of the Related Art
In a conventional recording apparatus, the leading edge of a piece of a recording medium, such as paper, is detected when the piece of recording medium is loaded from a paper supply port. The recording apparatus then sets the position of the recording paper leading edge at a predetermined position with respect to a recording line and carries out what is known as an initial printing position alignment operation. Also, if necessary, the recording apparatus detects a trailing edge of the piece of recording medium and carries out an operation to stop recording thereon.
Detection of the leading and trailing edges of the sheet of recording medium is conventionally carried out by including a switch in the paper feed path which switches on when it contacts the piece of recording medium and switches off when contact with the recording medium is broken. However, this method typically provides low detection accuracy. The low accuracy can be caused by factors such as the choice of a simple mechanical construction and method of signal processing in the recording apparatus.
When the end portion of a piece of a recording medium must be detected with a high level of accuracy, a different conventional method is utilized. A sheet end detector is used, which includes a light-emitting element which irradiates a piece of a recording medium with light and a light-receiving element which receives light reflected from this irradiated piece of recording medium. This light-receiving element is disposed in a position where it is possible to intercept light reflected from the piece of recording medium. The light receiving element generates a signal which changes when light is or is not reflected from the recording medium. The level of the signal indicates the level of light reflected from the recording medium. The signal is received by the sheet end detector and is compared with a previously set predetermined standard level.
It has been determined that such a conventional method has drawbacks. For example, as is shown in FIG. 6, a light beam SL is emitted from a light-emitting element SS of a sheet end detector. As light travels from light-emitting element SS, it will spread slightly. Therefore, when light beam SL reaches recording medium P, its diameter is slightly greater than the diameter of light-emitting element SS. In the same way, when the light beam SL is reflected from recording medium P, and is received by a light-receiving element (not shown), it will be even more diffused and its diameter will be even greater.
When piece of recording medium P moves in a direction towards a printing region, as indicated by arrow PD in FIG. 6, the amount of light reflected by the piece of recording medium changes as a leading edge Ps of medium passes through light beam SL. As this occurs, the signal level, that is, the detector voltage, changes in proportion to the amount of the recording medium which has been fed through beam SL, as shown in FIG. 7. However, the amount of reflected light is also dependent upon the reflectance ratio of the piece of recording medium. Because different media have different reflectance ratios, the rate of change of the detector voltage- that is, the slope of the signal depicted in FIG. 7, is influenced by the recording medium reflectance ratio. For example, a paper A and a paper B will cause the generation of detection voltages having different slopes. Finally, the detector voltage changes up to the saturation voltages Vm1 and Vm2, which are governed by the recording medium reflectance ratio, and afterwards becomes constant.
In operation, when the recording medium is being fed, the detector voltage will reach a predetermined standard level Vs. When voltage Vs is reached, the sheet end detector will determine that the leading edge of the recording medium has passed the position of beam SL. However, it has been determined that because of the different reflectance ratios of recording media paper A and paper B (FIG. 7), the leading edge of a recording medium P is identified at different positions in the printing path because Vs will be reached at an earlier location with paper A than with paper B. This difference in location is identified as error .DELTA.L where .DELTA.L is a measure of the difference in location where two different recording media were halted and the apparatus determined that the leading edge of each recording medium had been detected. Also, because of the diffusion of light noted above, the reflected light may reach the detector before the recording medium is in the proper position, thus resulting in a positioning error.
Because there are natural variations in the level of irradiation of the light-emitting element and in the detection sensitivity of the light-receiving element which make up the sheet end detector, in order to reduce the size of this error .DELTA.L in a conventional apparatus, it would be necessary to carefully select and employ detectors in which the characteristics are the same and the error is reduced. This results in the problem of increased component costs and complexity of the apparatus.
Accordingly, it is desirable to provide a detection method and apparatus which overcome deficiencies in conventional methods and apparatus.